1. Field of the Invention
The present invention relates to an optical waveguide for transmitting a surface plasmon-polariton wave. In the optical waveguide, a thin metallic film in which a surface plasmon-polariton wave is transmitted is formed to have a triangle-shaped cross-section, and a pair of dielectric layers interposing the metallic thin film are coupled to each other such that the opposing surfaces thereof come in contact with each other, the dielectric layers having a different refractive index. Such a structure allows a light-transmission distance to be enlarged.
2. Description of the Related Art
A surface plasmon is an oscillating wave which propagates along an interface between materials with dielectric constants having a reverse sign. In general, a surface plasmon exists at the interface between metal having the negative sign and a dielectric having the positive sign, and can be excited by electrons accelerated at high speed and light waves. An electromagnetic wave which is coupled to the surface plasmon so as to propagate is referred to as a surface plasmon-polarion (hereinafter, referred to as “SPP”) wave.
Since the wave vector of the surface plasmon is larger than those of peripheral materials, the SPP wave is bound to a metal surface. Therefore, it can be considered that the interface between metal and a dielectric is a two-dimensional optical waveguide with a vertical binding condition.
In view of the optical waveguide, a SPP wave to be generated at the interface between a metallic thin film and a dielectric is effectively bound, while a propagation distance is as short as dozens of μm in a visible-ray region. However, when the thickness of the metallic thin film is limited to several nm to dozens of nm such that a SPP wave propagating at the interface is coupled to the metallic thin film, long-range transmission of light can be realized. This is referred to as a long-range surface plasmon polariton (LR-SPP) mode. The field profile of the LR-SPP mode is widely distributed in the dielectric around the metallic thin film. Therefore, a propagation loss of light is small, and a coupling characteristic with optical fiber is excellent. Accordingly, the LR-SPP mode is applied to various optical-element fields.
In general, an SPP optical waveguide in which a metallic thin film is interposed is operated at the LR-SPP mode or an SR-SPP (short range surface plasmon-polariton) mode. In the LR-SPP mode, the metallic thin film is formed to have a thickness of less than dozens of nm such that light is propagated by a long distance. In the SR-SPP mode, light is propagated through a waveguide having a relatively small size.
When a metallic thin film having a finite cross-sectional area is used, a binding condition of SPP can be reduced into the three dimension, and an LR-SPP waveguide performing a similar action to a dielectric waveguide can be formed.
Therefore, the electric field of the LR-SPP is widely distributed in the dielectric around the metallic thin film such that the LR-SPP sensitively reacts to an optical change of the peripheral dielectric. Therefore, a long-distance transmission can be realized, and the LR-SPP is currently applied to an optical waveguide element which is used in optical modulators, switches, couplers, filters, and optical sensors.
The technical construction of the conventional optical waveguide for transmitting an SPP wave is disclosed in Japanese Unexamined Patent Application Publication No. 2005-114768. The structure thereof will be briefly examined, and the problems thereof will be described as follows.
FIG. 1 is a perspective view of the conventional optical waveguide for transmitting an SPP wave. As shown in FIG. 1, the optical wave guide 10 includes a dielectric substrate 11 having a positive dielectric constant and a metallic strip structure 12 formed on the surface of the dielectric substrate 11, the metallic strip structure 12 having a negative dielectric constant. As the diameter of light is reduced through the strip structure 12, the optical waveguide has a transmission distance of several to dozens of μm.
In the optical waveguide 10, a V-shaped groove 11a is formed on the surface of the substrate 11 by an ion-milling or dry-etching process. A metallic material is filled in the V-shaped groove 11a by a sputtering method. Then, a wide-range strip is formed on the surface of the substrate 11 including the upper surface of the metallic material filled in the V-shaped groove 11 such that the strip structure 12 having a metal layer 12a and a strip 12b integrated therein is formed, the metal layer 12a having a triangle-shaped cross-section.
In this case, the metal layer 12a having a triangle-shaped cross-section may be formed so as to be exposed to the upper portion of the strip 12b. 
In such a structure, an electric field is concentrated in the metallic layer 12a having a triangle-shaped surface. Therefore, as the diameter of light to be propagated along the strip 12b can be narrowed, the diameter of the light can be bound to less than 200 nm. Further, a transmission distance of about 10 μm, which is approximate to a transmission distance of surface plasmon-polariton wave in the metallic thin film, can be achieved.
In the conventional optical waveguide for transmitting an SPP wave, however, since an electric field is concentrated in the metal layer 12a having a triangle-shape cross-section, a transmission distance thereof is no more than dozens of μm at most, even though the diameter of light is limited to less than 200 nm. Therefore, there are difficulties in using the optical waveguide structure as a sensing element which is applied to optical modulators, switches, couplers, filters and optical sensors.